150. Evaluate Reverse Polish Notation QuestionEditorial Solution

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Some examples:

  ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

stack只存数字，遇到operator则pop两个数字并进行计算，然后将结果存stack。最后stack的第一个元素就是答案。注意 - 与 / 的差别， 先pop的值在算符右边。

public int EvalRPN(string[] tokens) {
        Stack<int> stack = new Stack<int>();
        for(int i = 0;i< tokens.Count();i++)
        {
            if(tokens[i] == "+")
            {
                stack.Push(stack.Pop() + stack.Pop());
            }
            else if(tokens[i] == "-")
            {
                stack.Push(-1*stack.Pop() + stack.Pop());
            }
            else if(tokens[i] == "*")
            {
                stack.Push(stack.Pop() * stack.Pop());
            }
            else if(tokens[i] == "/")
            {
                int a = stack.Pop();
                int b = stack.Pop();

                stack.Push(b / a);
            }
            else
            {
                stack.Push(Int32.Parse(tokens[i]));
            }
        }
        return stack.Peek();
    }